Patent Document 1 discloses a distributed power supply device that supplies alternating-current power to a home load in conjunction with a commercial electric power system. The power supply device described in Patent Document 1 is installed in areas where an abnormality in input voltage frequently occurs due to momentary interruption (such as momentary voltage drop or momentary power failure) of the commercial electric power system or other causes. With the power supply device, if the pattern of occurrence of abnormality type and characteristic values upon each detection of an abnormality matches a pre-stored time-series occurrence pattern of abnormality type and characteristic values, it is determined that a device failure has occurred. After the determination, the device is restarted or completely stopped.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2009-290918.
Some power supply systems required to have high reliability are often designed with redundancy to allow selection of a plurality of commercial electric power systems in anticipation of an interruption of power supply from a commercial electric power system due to a power failure or other causes. A mechanical relay switch is used to switch between the commercial electric power systems. Switching a mechanical relay switch many times can potentially lead to reduced product lifetime or reliability. For this reason, it is desired to avoid frequent switching of the mechanical relay switch. If, to this end, the number of times to execute abnormality determination is reduced while maintaining the amount of time spent on each single abnormality determination, this can potentially lead to reduced accuracy of abnormality determination, making it impossible to take an appropriate measure in the event of an abnormality.